Every 2x converter in this world is "worse" than its 1.4x counterpart. Converters magnify the image produced by the lens. So, the more you magnify the better you will see any weaknesses of the lens.That's why Olympus recommends to use only the best SHG lenses with their EC-20. Some are happy to use theirs with the 50-200mm but I never managed to get flawless sharpness with this combination.

The EC-20 works well on my 300mm telephoto lens, though. This is due to the fact that wide open is an aperture of 5.6 (it says 5.7 in the viewfinder) and also to a stable tripod and a shutter operating technique inspired by the use of firearms (long ago :-)).